1. Field of the Invention
The present invention is concerned with the field of induction plasma torches and relates more specifically to a plasma torch of which the performance is improved by permeating liquid through the plasma confinement tube. Vaporization of the permeating cooling liquid enables formation of plasmas in particular of water vapor (steam) but also of other vaporizable liquids.
2. Brief Description of the Prior Art
Induction plasma torches have been known since the early sixties. Their basic design has however been substantially improved over the past thirty years.
The basic concept of an induction plasma torch involves an induction coupling of the energy into the plasma using an appropriate induction coil. A gas distributor head is used to create a proper flow pattern into the region of the produced plasma, which is necessary to stabilize the plasma confined in a tube usually made of quartz, to maintain the plasma in the center of the coil and protect the plasma confinement tube against damage due to the high heat load from the plasma. At relatively high power levels (above 5-10 kW), additional cooling is required to protect the plasma confinement tube. This is usually achieved through deionized water flowing on the outer surface of the tube.
Numerous attempts have been made to improve the protection of the plasma confinement tube. These tentatives are concerned with the use of (a) a protective segmented metallic wall insert inside the plasma confinement tube (U.S. Pat. No. 4,431,901 (Hull) issued on Feb. 14, 1984), (b) porous ceramic, permeable to gas, to construct the plasma confinement tube (J. Mostaghimi, M. Dostie, and J. Jurewicz, "Analysis of an RF induction plasma torch with a permeable ceramic wall", Can. J. Chem. Eng., 67, 929-936 (1989)), (c) radiatively cooled ceramic plasma confinement tubes (P. S. C. Van der Plas and L. de Galan, "A radiatively cooled torch for ICP-AES using 1 liter per min of argon", Spectrochemica Acta, 39B, 1161-1169 (1984) and P. S. C. Van der Plas and L. de Galan, "An evaluation of ceramic materials for use in non-cooled low flow ICP torches", Spectrochemica Acta, 42B, 1205-1216 (1987)), and (d) a high velocity water-cooled ceramic confinement tube (U.S. Pat. No. 5,200,595 (Boulos et al.) issued on Apr. 6, 1993). These attempts each present their respective limitations and shortcomings.
The use of a segmented metallic wall insert to improve protection of the plasma confinement tube present the drawback of substantially reducing the overall energy efficiency of the plasma torch.
It has been found that a plasma confinement tube made of porous ceramic material permeable to gas offers only limited protection. It also requires a large flow rate of transpiration gas to be effective. This results in a substantial reduction of the specific enthalpy of the plasma gas at the exit of the torch.
Concerning the radiatively cooled confinement tubes, their ceramic materials must withstand the relatively high operating temperatures, exhibit an excellent thermal shock resistance and must not absorb the RF (Radio Frequency) field. Most ceramic materials fail to meet with one or more of these stringent requirements.
Although the use of a high velocity water flow established in a thin annular chamber (U.S. Pat. No. 5,200,595) constitutes a major advance for cooling the confinement tube, its efficiency is limited since the water is applied to the outer surface of the confinement tube only.
British patent N.degree. 1,066,651 (Cleaver) dated Apr. 26, 1967, proposes the use of a plasma torch having a porous confinement tube to produce metal or metalloid oxides by the vapour phase reaction of metal or metalloid halides with oxygenating gas. A gas or vaporisable liquid is transpired through the confinement tube to prevent, during the process, part of the metal or metalloid oxide produced to be deposited on the inner wall of the confinement tube in the form of an encrustation which can be hard and difficult to dislodge. This patent mentions that the transpired gas or vaporisable liquid has the further useful effect of cooling the porous confinement tube through which it is transpired.
All of the above described prior art methods of cooling the confinement tube of an induction plasma torch are unsuitable for substantially reducing the amount of plasma gas required to operate the plasma torch. Also, they do not allow such a torch to operate with condensable vapours such as water vapour (steam) without resorting to use of high temperature coolants for the cooling of the plasma torch.